The present invention relates to electrodes for electrolytic capacitors and, more particularly, to a method for producing such electrodes, particularly anodes but also cathodes, having increased surface area, and the electrodes so produced.
An electrolytic capacitor is a capacitor in which one plate is metallic and the other plate is an electrolyte. Intervening between the two plates is a dielectric consisting of a surface oxide coating on the metal plate. Conventionally, the metal plate on which the dielectric coating is formed is referred to as the anode. The term xe2x80x9canodexe2x80x9d is used herein to refer both to the metal plate itself and to the combination of the metal plate with the dielectric coating. It will be clear from the context which meaning of xe2x80x9canodexe2x80x9d is intended. A transition between ionic conduction in the electrolyte and electronic conduction in an external circuit is provided by a second metal plate which is referred to herein as the cathode. The anode and the cathode are referred to herein collectively as electrodes. Typically, the anode and the cathode are thin foils.
Typically, the metal of the anode is a valve metal, i.e., a metal which, when oxidized, allows current to pass if used as a cathode but opposes the flow of current when used as an anode. Examples of valve metals include magnesium, thorium, cadmium, tungsten, tin, iron, silver, silicon, tantalum, titanium, aluminum, zirconium and niobium.
As is the case with capacitors generally, the capacitance of an electrolytic capacitor is proportional to the surface areas of its two plates. Conventionally, the surface areas of the foils are increased by etching. In the case of very thin foils, this has the disadvantage of weakening the foils mechanically, making it difficult to fabricate electrolytic capacitors by high speed winding.
Recently, vacuum deposition has been proposed for increasing the surface areas of foil electrodes. Kakinoki et al., in U.S. Pat. No. 4,970,626, report the vacuum deposition of titanium on aluminum foil, at an angle, to produce a titanium surface with a rough, columnar structure. The disadvantages of this method of increasing the surface are of a foil electrode include the additional cost of working with two metals and the decrease in stability associated with an intermetallic potential. Drake, in U.S. Pat. No. 4,309,810 also teaches the vacuum deposition of a metal vapor at a low angle onto a foil substrate, and presents an example of the deposition of aluminum on aluminum to give a columnar structure. Drake""s foil has been found to be too brittle for use in electrolytic capacitors: it breaks when it is rolled into a cylindrical roll, the standard shape of an electrolytic capacitor. Neumann et al., in German Patent No. 4,127,743, report the vacuum deposition of aluminum on aluminum in a low pressure oxygen atmosphere, to give a surface structure of columns of aluminum separated by aluminum oxide. Allegret et al., in U.S. Pat. Nos. 5,431,971 and 5,482,743, also report the codeposition, under a low pressure oxidizing atmosphere, of a mixture of aluminum and aluminum oxide. Such mixed Alxe2x80x94Al2O3 surfaces are more robust mechanically than pure aluminum surfaces; but electrolytic capacitors incorporating them are known to have relatively high resistive losses and relatively low stability over time. In addition, the presence of both aluminum and large quantities of aluminum oxide in the surface of the foil makes difficult and less effective both stabilization by subsequent conventional chemical or electrochemical treatments and structure coarsening by subsequent annealing.
Having formed a foil electrode with high surface area, if the electrode is to be used as an anode, its surface must be oxidized. Conventionally, this is done by electrolytic anodization, in which the electrode is used as an anode in an electrochemical cell. Recent patents in this art include U.S. Pat. No. 4,537,665 to Nguyen et al., U.S. Pat. No. 4, 582,574, to Nguyen et al., and U.S. Pat. No. 5,643,432 to Qiu. The thicker the oxide layer, the lower the capacitance of the electrolytic capacitor, but the higher the working voltage of the dielectric. For high voltage (upwards of 100V) applications, the dielectric layer is relatively thick, and tends to bridge over fine surface features, reducing the effective surface area of the anode.
Two other phenomena tend to reduce the effective surface areas of anodes made by electrolytic anodization. One is that in the course of the anodization process, oxygen and hydroxide ions migrate from the metal-dielectric interface into the metal, while metal ions migrate from the metal-dielectric interface into the dielectric. The other is that sharp points on the metal surface are characterized by high local electric fields, which accelerate the electrolytic process. Both of these phenomena tend to smooth out irregularities in the metal-dielectric interface.
There is thus a widely recognized need for, and it would be highly advantageous to have, improved methods for creating mechanically robust foil electrodes of high surface area.
According to the present invention there is provided a method for increasing the surface area of a substrate, including the steps of: (a) placing the substrate in an inert atmosphere having a pressure of between about 10xe2x88x923 torr and about 10xe2x88x922 torr; and (b) evaporating a valve metal onto the substrate under the inert atmosphere, thereby imparting a surface structure to the substrate.
According to the present invention there is provided a method of forming a dielectric layer on a substrate, including the steps of: (a) forming a discontinuous layer of an oxide of a first valve metal on the substrate; and (b) electrolytically anodizing the substrate subsequent to the forming of the discontinuous layer.
According to the present invention there is provided a method of forming a dielectric layer on a substrate, including the steps of: (a) evaporating onto the substrate a substance selected from the group consisting of valve metals and oxides thereof, thereby: (i) if the substance is a valve metal, forming a layer of an oxide of the valve metal on the substrate, and (ii) if the substance is an oxide of a valve metal, forming a layer of the oxide on the substrate; and (b) electrolytically anodizing the substrate subsequent to the forming of the oxide layer.
According to the present invention there is provided an article of manufacture including a valve metal having a fractal-like surficial structure.
According to the present invention there is provided an electrode including: (a) an electrically conductive substrate; and (b) a discontinuous layer, of an oxide of a first valve metal, on a surface of the substrate.
According to the present invention there is provided an anodized electrode, including: (a) an electrically conductive substrate; and (b) a dielectric coating, on a surface of the substrate, having a bimodal morphology.
The methods of the present invention are suited to increasing the surface area of any substrate, and to forming a dielectric layer on any substrate. Nevertheless, the primary application of the methods of the present invention is to the production of electrodes for electrolytic capacitors, and the methods of the present invention are illustrated herein with reference to this primary application.
According to a first aspect of the present invention, a valve metal such as aluminum is evaporated onto a foil surface in a low pressure atmosphere of an inert gas, preferably in the presence of a still lower, by between one and two orders of magnitude, pressure of oxygen. The inert gas may be any gas that does not react with the valve metal under process conditions, including noble gases such as helium and argon. In the experiments reported herein, the valve metal is aluminum and the inert gas is nitrogen.
The valve metal surface thus formed has a fractal-like structure, with a fractal dimension greater than 2. The scope of the present invention includes articles of manufacture in general, not just foil electrodes, that have valve metal surfaces or interfaces with such fractal-like surficial structures. This surficial structure may be on the surface of the article of manufacture, as in the case of a foil cathode of the present invention. Such a cathode has a higher capacitance than prior art cathodes of similar bulk composition and overall dimensions. Alternatively, the surficial structure may be an interface in the interior of the article of manufacture, as in the case of a foil anode, either a foil anode of the present invention or a foil anode formed by conventional anodization of a foil cathode of the present invention. In such an anode, the fractal-like valve metal surficial structure is covered by an oxide dielectric. In foil anodes of the present invention, the fractal-like surface structure allows the deposition of relatively thick dielectric layers while preserving the high surface area of the metal surface.
Because the present invention increases the surface area of a valve metal foil by adding material, rather than by removing material, as in conventional etching, the present invention allows the use of thinner foils in applications such as electrodes for electrolytic capacitors.
The scope of the present invention includes articles of manufacture in which the fractal-like surficial structure is formed on any substrate, including nonmetallic and even insulating substrates. Similarly, the scope of the present invention includes the evaporation of valve metals onto all suitable electrodes, including nonmetallic electrodes such as electrodes made of electrically conducting polymers. Nevertheless, the focus herein is on electrodes made of valve metal foil.
According to a second aspect of the present invention, a foil electrode is prepared for anodization by depositing thereon a surface layer, preferably a discontinuous surface layer, of a valve metal oxide. If the surface of the foil electrode is rough, then to the extent that sharp points on the surface are covered by the oxide thus deposited, those points are isolated from the surface-area-reducing effects of subsequent electrolytic anodization. On a smooth foil electrode surface, or on smooth portions of an otherwise rough foil electrode surface in between the sharp points, the subsequent electrolytic anodization takes place predominantly on portions of the surface left exposed after the deposition, thereby actually increasing the area of the metal-dielectric interface. The scope of the present invention includes the anode thus formed, which is distinguishable from prior art anodes because the dielectric layer thereof has a bimodal morphology: the non-electrolytically formed, preferably discontinuous layer is homogeneous, whereas the electrolytically formed layer is increasingly porous towards its outer surface, the pores being predominantly cylindrical and perpendicular to the outer surface.
The non-electrolytically formed surface oxide layer preferably is deposited by evaporation, either of the valve metal in the presence of low pressure oxygen, or of the valve metal oxide itself. Under both aspects of the present invention, it is most preferable that the metal of the foil electrode be the same as the valve metal deposited thereon as metal or as oxide. Optionally, a (preferably discontinuous) surface layer of an oxide of a second valve metal is deposited over the first surface layer before the electrolytic anodization.